I use two metal rods 3/16 - 1/4 inch dia 6mm by 750mm long with a 150mm right angle bend handle.
You put both arms out full length in front of you about 225mm apart. Put the handle into a clenched fist but allow the rods to freely rotate. Hold the rods out horizontal 225 mm apart and walk over where you think the pipe is. When you get over the pipe the rods will freely cross over. I do it both ways to check where the pipe is. I have done building sites this way to find land drains and also under roads to find drains without problems. Once you have the drain line you can probe for the cover. Check for lateral drains coming in to see if the IC and cover is over it.
I showed on chap how to do it and he could differentiate between the services although I cant.
I have done it with wall ties and the 73mm paper clips.
Easier with the battery version, but not as cheap.
Stand back for the flack.
Mine Lectures & Walks available for around Huddersfield